1. Field of the Invention
The present invention relates to an electrochemical cell, where the electrodes, namely the anode and cathode, accompanied by the respective interposition of separators, are disposed in a circular arrangement in alternate layers from the inside toward the outside.
The electrochemical cell of the present invention is related in particular to a lithium cell having a lithium anode for high-power use, with the cathode preferably being a MnO.sub.2 cathode; however, other materials could also be used for the cathode.
The "circular arrangement" of the cell refers either to a bobbin construction or a winding construction. In a bobbin construction, the electrodes are embodied as cylindrical tubes and, while being separated by a similarly cylindrical separator, are telescoped within one another. In a winding construction, the components, including the electrodes and separators, are superimposed over one another as long strips and are rolled to form a tight helical winding. The highest electrical outputs can be achieved with a winding construction.
The use of lithium as anode material has the advantage that this element has the greatest negative potential of all elements, as well as the highest capacity per specific weight of all anode materials. For this reason, lithium cells form high output cells. The lithium anode can be combined with a cathode of, for example, manganese dioxide (MnO.sub.2), in other words, with a cathode of solid material. Liquid cathodes would also be conceivable.
One problem with electrochemical cells of the aforementioned general type is the outermost electrode layer. Thus, for example with lithium cells, especially of winding construction, more lithium is provided in the outermost lithium layer than can be discharged via the adjacent counterelectrode. Thus, the capacity of the outermost lithium layer cannot be completely tapped since no adequately dimensioned counter-electrode is available. However, an optimum efficient utilization of the available volume, especially when cathodes of solid material are used, is possible only if the capacity of the anode and cathode in the outer layer of an electrode winding are coordinated with one another. The same situation naturally also applies for a bobbin construction.
An excess of free lithium in a discharged cell represents an acute safety risk if the cell is mishandled, for example due to polarity reversal or charging. This can lead to explosion of the cell, which of course represents a very great danger that an accident will occur. This danger exists in particular if several cells are connected together in a battery. In such a situation, the cell having the least capacity at the conclusion of discharging has its polarity reversed by the remaining cells. In order to preclude the aforementioned danger of an accident occurring, it has up to now been customary for the faulty current that results from the polarity reversal in batteries having lithium high-output cells to be partially taken over by a diode that is connected in parallel. However, this protective measure in the form of a diode is technically complicated.
2. Description of the Prior Art
German Offenlegungsschrift 33 01 297 discloses an electrochemical cell of winding construction having a lithium anode comprised of two superimposed anode strips. Embedded between the anode strips is a copper strip that is intended to prevent damage to the cell during irregular operation. Electrochemical cells are furthermore known where a separator of micro glass fibers is disposed between the electrodes, with the cells having an electrolyte. With a separator of micro glass fibers, use is made of the positive properties with regard to the chemical resistance as well as the electrical properties that such separators provide. Unfortunately, the known fabrics (papers) of micro glass fibers, as they are used for separators, also have drawbacks. On the one hand, such separators have an only slight mechanical stability and strength, and can therefore be loaded only slightly. The result can be that in winding cells having cathodes that increase in volume during discharge of the cell, as is the case, for example, with MnO.sub.2 cathodes, the separators can become damaged during the discharge. This can lead to uncontrollable consequences, so that due to the non-achieved necessary strength factors of the separators, the safety is endangered. On the other hand, due to the inadequate tightness or sealing of the heretofore known separators, the danger exists that the anode material will press through the existing faulty locations. This is in particular the case where soft lithium is the anode material.
For this reason, for example with lead storage batteries, it has already been proposed to strengthen the micro glass fibers of the separators via a binder; unfortunately, this binder is unstable in organic solvents.
German Offenlegungsschrift 33 23 233 furthermore proposes a separator of micro glass fibers that are strengthened via a binder, for example in the form of polyvinyl alcohol. In this case, however, the cells have liquid positive electrodes.
A further problem of electromechanical cells, and in particular lithium cells, is represented by the high output range, especially during operation. Up to now, lithium batteries with organic electrolytes have been usable only to a limited extent at low temperatures of up to -30.degree. C., since the voltage state and derivable capacity greatly drop at temperatures below 0.degree. C., especially at high loads. As a result, the advantages of lithium cells having non-toxic organic electrolytes cannot be used in some applications.
A further problem exists in the provision of a stable separator binder/electrolyte combination. This is so because it has been shown that certain separator binders often cannot be used in cells having organic electrolytes.
German Patent 23 04 424 discloses an electrochemical cell having an electrolyte that provides a mixture of dioxolane with ethylene carbonate.
It is an object of the present invention, starting with an electrochemical cell, especially a lithium cell, of the aforementioned general type, to increase the safety of the cell without great technical complexity or expense.